The present invention relates to cups formed of paperboard and, in particular to methods and apparatus for making a cup bottom that is to be attached to a sidewall of the cup.
In the manufacture of paper cups, i.e. cups made of paperboard material which could optionally be coated with a foamed substance, it is conventional to form a cup bottom and then wrap a cup sidewall around the bottom and attach the sidewall to the bottom by an adhesive.
A longitudinal sectional view through a conventional paper cup 10 is depicted in FIG. 1. As can be seen therein, the cup includes a bottom 12 and a sidewall 14 attached thereto. The bottom 12 comprises a disk-shaped base 16 and a cylindrical lip or skirt 18 projecting from an outer periphery of the base 16. The sidewall 14 is wrapped around the circumference of the lip 18, and an end 20 of the sidewall is folded over the free edge of the lip 18. The lip 18 thus becomes sandwiched between portions of the cup sidewall and is bonded thereto by an adhesive.
Prior to being wrapped with the sidewall, the cup bottom is formed by passing a paper web across a cutter which cuts out a circular blank. Then a draw pushes the blank through an opening having a smaller diameter than the blank. Hence, an outer periphery of the blank is bent over to form the lip, the lip being squashed as it travels through a gap between the draw and a surface of the opening. A conventional apparatus for the manufacture of the cup bottoms is disclosed in Budziszewski U.S. Pat. No. 5,624,367, the disclosure of which is incorporated by reference herein.
It will be appreciated that the lip-forming outer periphery of the blank has a first circumference before the bending, and a smaller circumference after the bending. That means that there is extra paper material after bending, and that extra paper material produces pleats 21, 22 which project from the surface of the lip (see FIG. 2). The pleats extend axially (i.e., in a vertical direction when the cup sits upright). Some of the pleats 21 project by a small distance from the lip surface (i.e. they have a very a short height) and do not present problems, because they will become flattened when the lip is compressed in the gap, as shown in FIG. 2. Other pleats 22, however, are tall enough to become folded over when the lip is compressed in the gap. Such folded-over pleats can produce leakage paths between the lip and the cup sidewall which permit liquid to leak from the cup.
The apparatus disclosed in U.S. Pat. No. 5,624,367 attempted to deal with that problem by producing radial score lines in the outer periphery of the blank prior to the bending. The score lines constitute pre-weakened regions of the blank. It was anticipated that the pleats would be formed in a controlled manner along the score lines, and that the problem of folded-over pleats would be eliminated. That proposal has not met with complete success, because folded-over pleats are still formed in the cup bottom.
It is, therefore, an object of the present invention to produce cup bottoms having only pleats which are not tall enough to become folded-over when the bottom lip is compressed.